The present invention generally relates to a class of bearings identified as touchdown or backup bearings used to selectively support rotor systems having magnetic bearings as the primary support bearings. More particularly, the present invention is directed to a touchdown bearing assembly having a unique actuator assembly capable of quickly and repeatably engaging and disengaging the touchdown bearing with the shaft with a minimum of overall space and weight.
Magnetic bearings are often employed to support gas turbines and other high speed rotating machinery because of their unique ability to suspend and balance the rotor without the need for metal-to-metal contact with a rolling bearing. However, in the event of a fault or instability or even inoperability of the magnetic bearings, it is imperative that back-up rolling bearing(s) immediately engage and support the rotating shaft to avoid damage to the machine due to direct rubbing contact between rotating and static hardware.
The majority of touchdown bearings in use today employ a passive engagement scheme, in which the inner bore diameter of the bearing has a radial clearance to the rotating shaft surface when in the disengaged position. During startup and shutdown, or in the event of a magnetic bearing fault, the rotating shaft drops onto the touchdown bearing. In order to prevent damage to the machine, the radial touchdown-bearing gap must be smaller than the radial clearance between the rotor and the static structure. On gas turbine engines, the operating blade to shroud tip clearance can be 0.006 inches or less, leaving little radial clearance for a passive engagement scheme using radial clearance between the bearing and the rotor shaft.
In an effort to overcome the problems associated with the radial clearance approach, U.S. Pat. No. 5,747,907 issued May 5, 1998 to Miller, suggests that a conical feature be used for centering the rotor to prevent whirl, and allow the rotor to safely spin down. Miller is directed to supporting flywheel energy storage devices that do not produce any significant axial force on the rotating shaft. To engage the bearing, Miller suggests that either a spring or a piston may be employed. It would be prohibitive to employ either of these actuators with a machine subjected to the type of thrust forces affecting gas turbine rotors. If a spring actuator of the type suggested by Miller were employed in a gas turbine machine, the spring would have to be unduly large to overcome the thrust forces that are tending to compress it. In addition, the mechanism for holding the touchdown bearing system in its disengaged position would necessarily have to be large in mass with a correspondingly slow reaction time due to the large electromagnetic force required to keep the touchdown bearing in the disengaged position against the high force of the engagement springs. Such a system would also have a large continuous electrical requirement to supply the electromagnet used to keep the touchdown bearing disengaged. Alternatively, if a piston actuator were employed as suggested in Miller, there would be a slow reaction time due to the limitations in pumping fluid into the piston chamber as well as due to the mass of the hardware.
In a further known assembly suggested in U.S. Pat. No. 4,629,261 issued Dec. 16, 1986 to Elermann et al., a rolling backup bearing assembly is engaged via a spring with an electric release mechanism. A purely axial spring is employed to move the bearing in the axial direction into engagement with the rotor. The system described in Elermann can only tolerate axial thrust loads that are below the spring force. Any higher thrust force would allow axial movement of the backup bearing and potentially allow rubbing between the rotating and static hardware. To assure that the bearing would not move when subjected to large axial forces as would occur with gas turbines, Elermann would have to employ a very large spring as well as a massive electromagnetic release mechanism, again consuming significant electrical power, and slowing down the reaction time due to the high mass.
There is clearly a need for a backup or touchdown bearing assembly that quickly engages the rotating gas turbine shaft without requiring a massive spring actuator or massive electromagnetic release mechanism. If the backup bearing actuator and release mechanism does not have to directly counteract the large axial thrust forces produced by the rotating gas turbine shaft, the mechanism could be made small and lightweight, allowing it to achieve the desired quick response times with reduced power consumption.
In one aspect of the present invention, a rapid engagement touchdown bearing and actuator ring assembly includes a rotating bearing movable in an axial direction into and out of surface contact with a rotating shaft which may be subject to strong axial forces, i.e., a gas turbine shaft. The actuator ring assembly includes a pair of ring members positioned adjacent the bearing assembly and capable of both relative rotational and axial motion. As one of the rings moves axially, it engages and moves the bearing in an axial direction against the action of a restraining spring assembly until a beveled surface on the bearing engages a similar surface on the rotating shaft. One or more control actuator spring(s) cause relative rotation of the actuator rings in a first direction until protuberances extending from one of the rings align with confronting protuberances extending from the other ring, causing the rings to wedge-apart, biasing the bearing assembly into direct contact with the rotating turbine shaft.
To disengage the touchdown bearing from the rotating shaft, at least one disengagement actuator is energized which causes reverse relative rotation of the rings in a second, opposite direction until the protuberances on the rings are out of alignment with each other, negating the wedging pressure between the rings, and thus allowing the restraining spring(s) to rapidly move the touchdown bearing assembly in the reverse axial direction, out of engagement with the rotating shaft.
Preferably the protuberances mounted on the face of one of the rings include a series of circumferentially-spaced balls or rollers while the protuberances mounted on the confronting face of the other ring include a corresponding series of circumferentially-spaced, incline ramps. Relative rotational movement of the rings in the first predetermined direction causes the balls or rollers to progress up the ramps, wedging-apart the rings. Rotation of one of the rings of about only 100-150 relative to the remaining ring is needed to align the protuberances and reach maximum ring separation of about 0.010 to 0.020 inches of the confronting ring faces. Such movement can take as little as about 2 milliseconds. The protuberances attached to the rings were designed to assure the rings remain in their wedged-apart positions without further assistance from the actuator control spring(s), even when subjected to significant axial thrust forces. As a result, the actuator ring assembly of the present invention requires a much smaller actuator control spring(s) than would otherwise be necessary. The invention described above can be made to function on one individual touchdown bearing, engaging it to support one end of a rotating shaft, providing close radial support and reaction of any axial rotor thrust loads, or the system can be adapted to engage two or more touchdown bearings to provide fast-responding touchdown bearing support at both ends of the rotor.
In another aspect of the invention, a pair of separate touchdown bearing assemblies, each having at least one beveled edge portion, may be wedged-apart in opposite axial directions, making contact with separate, beveled surfaces of the rotating shaft. A ring fixed against rotation and yet axial movable may be associated with one touchdown bearing assembly and a ring capable of both rotating and axial movement may be associated with the other touchdown bearing. One of the rings is preferably restrained against rotation while both rings are capable of axial movement. When the actuator ring assembly undergoes relative rotation, the protuberances on the confronting faces come into alignment and the rings are wedged-apart in opposite axial directions. Each ring engages a separate touchdown bearing, eventually pressing the touchdown bearings in opposite directions into engagement with the rotating shaft at two separate locations
In still another aspect of the invention, a first touchdown bearing assembly can be brought into engagement with a rotating shaft by axial movement of the rotating ring as discussed above, with a separate restraining spring being compressed as the rotating shaft moves an axial distance sufficient to bring a second touchdown bearing assembly into contact with the rotating shaft.
In a yet further aspect of the invention, an electric gear motor may be employed in a disengagement actuator, causing relative rotation of the rings in the opposite direction to negate the wedging pressure and allow the restraining spring(s) to bias the touchdown bearing out of contact with the rotating shaft. Alternatively, the gear motor may be replaced by an electric jackscrew, a hydraulic piston and cylinder using oil, fuel or a dedicated hydraulic fluid. In another aspect of the invention, a pneumatic piston and cylinder may be employed in the disengagement actuator.